The present invention concerns a zirconium-based alloy, suitable for use in a corrosive environment where it is subjected to increased radiation and comprising 0.5-1.6 percentage by weight Nb and 0.3-0.6 percentage by weight Fe. The invention also concerns a component in a nuclear energy plant, which comprises an alloy of the mentioned kind.
According to the prior art it is known to provide, in a nuclear energy plant, a component which comprises a zirconium-based alloy of the above-mentioned kind. Such an alloy has the advantage of fulfilling the requirements which are demanded on mechanical as well as corrosion properties of a material which in a corrosive environment is subjected to an increased radiation, in particular neutron radiation of the fast neutron kind.
Thanks to its relatively high Fe-content it is possible through a suitable heat treatment, comprising annealing and quenching, to obtain secondary phase particles consisting of Zr, Fe and Nb in a matrix of a-phase of the zirconium-based alloy. By a suitable choice of the heat treatment variables time and temperature it is furthermore, with given contents of the included alloying materials Nb and Fe, possible to control the size of and the distribution of the secondary phase particles. The secondary phase particles may have a positive effect on the corrosion resistance of the alloy. It is therefore important to optimize the distribution of and the size of the existing secondary phase particles. It is thereby highly important to find a suitable composition of the alloying elements included in the alloy.
The document U.S. Pat. No. 5,560,790 describes a zirconium-based alloy which comprises 0.5-1.5 percentage by weight Nb, 0.9-1.5 percentage by weight Sn and 0.3-0.6 percentage by weight Fe. Furthermore, this alloy comprises 0.005-0.2 percentage by weight Cr, 0.005-0.04 percentage by weight C, 0.05-0.15 percentage by weight O, 0.005-0.15 percentage by weight Si and the rest Zr. Thereby a microstructure is achieved in the material which includes particles of the kind Zr(Nb,Fe)2, Zr(Nb,Cr,Fe) and (Zr,Nb)3Fe. These secondary phase particles give the material good corrosion properties and good mechanical properties. Thanks to the high Fe-content, precipations of β-Nb-phase are avoided, which would have a negative influence on the resistance of the material against local corrosion attacks.
Sn is said to have a high solubility in the α-phase and will therefore, when it is present to the given amount, be dissolved in the α-phase and contribute to improved corrosion properties and mechanical properties of the same. It is pointed out that a too low content of Sn (below 0.9 percentage by weight) in the material influences the tensile strength of the material both in the long and in the short term. Furthermore, such a low Sn-content suppresses to a smaller extent a negative effect of a possible nitrogen incorporation on the corrosion resistance of the material. A Sn content above 1.5 percentage by weight influences the susceptibility of the material to working and in particular to cold working.
It is mentioned that Si and C contribute to a reduction of the size of the particles and to bring about a structural homogeneity in the material.
Oxygen is said to contribute to a finer structure of the material and is also used as a means for reinforcing the material through the solid solution, a so-called “solid solution strengthener”.
Nb is said to contribute to the strength properties of Zr and increases the corrosion resistance of the alloy by forming secondary phase particles together with Zr and Fe.
It is furthermore pointed out that with a Nb-content below 0.5 percentage by weight of the material, a Fe-content below 0.3 percentage by weight and a Cr-content below 0.005 percentage by weight, the total portion of secondary phase particles of the above-mentioned kind in the α-zirconium matrix of the end product is considerably lower than 60 percentage by volume of the total amount of iron-containing secondary phase particles, which results in that the corrosion resistance of the material is negatively influenced. With a Nb-content above 1.5 percentage by weight, a large number of large particles of β-Nb phase are formed in the material, which also reduces the corrosion resistance of the same.
It is also mentioned that a Cr-content above 0.2 percentage by weight may result in the formation of binary intermetallic compounds of Zr—Cr, which has an opposite i.e. negative, influence on the workability and the tensile strength of the material.